I don't believe the forum has the ability to mail non-members. There's certainly nothing in the interface to allow that.

I would start by looking at the mail logs of the server itself. First see if there is actual mail being sent by your server. If it is sent, then it would show what user sent it, and that would show you that it was (or wasn't) sent by FUDforum. I'm not sure how long you keep you mail logs on that server, or when this alleged spamming happened, but hopefully you still have the logs from the timeframe when this happened and you can look at them. If you don't have the logs, it would be useful to see the full email details of even one of the emails that cause the issue.

I say this because it's also likely that no actual mail came from your server. Someone could have spoofed your server and/or email addresses on that server.

May I ask what kind of system you are running this on? Linux/windows? What kind of mail system does it use? Do you have root/admin privileges on that server?